Add some unit tests for CI_Input
Add a unit test for CI_Utf8::_is_ascii()
Disable HHVM mysqli and pgsql tests

mysqli and PostGre support isn't going to be in a good state for a
while.
Merge pull request #2823 from fredemmott/hhvm-travis

Disable HHVM mysqli and pgsql tests
Add CI_Model unit test
Unit tests: Full code coverage of Benchmark class
Minor changes related to CI_User_agent

Fixed a bug where both accept_charset() and accept_lang() improperly parsed headers
if they contained spaces between data separators (which is valid).

Also made is_referral() testable by replacing its static cache var with a class
property and added some more unit tests for the library as a whole.
Add autoloading library aliasing support (supersedes PR #2824)